Liver failure leads to the accumulation of toxins such as ammonia in the blood of patients. Apparatuses for removing these blood toxins (also called liver assist systems) have been developed and are grouped into passive or bioactive devices. Passive devices generally remove the toxins by hemodialysis, hemoperfusion, or plasma exchange, while bioactive devices can include living cells which remove or convert the blood toxins. An example of a bioactive liver assist system containing hepatocytes is described in U.S. Pat. No. 5,643,794.